Surfaces of many different materials are coated in a variety of applications for aesthetic reasons and for protecting the surface against physical and environmental damage. It is desirable to determine the thickness of the coating applied to the surface for a number of reasons. It is also desirable to obtain as accurate a determination of thickness as possible, which, in the past, has been difficult for extremely thin coatings
For example, it may be desirable to apply a minimum, predetermined thickness of a coating, such as a paint or primer material on a surface, since optimal adhesion of the paint coating is generally a function of the paint thickness. The paint or primer may be opaque. In addition, applying at least a minimum thickness of paint or primer to a surface ensures that any underlying visual features on the surface do not appear through the coating. This may be important in instances where projecting a professional image to customers is important, and to instill a feeling of customer confidence, such as through company signage or commercial airline tail art. This is also important for ensuring that a product, such as a motor vehicle or an airplane, displays a finish quality that is favored by the customer.
Further, applying at least a minimum thickness of the paint or primer may also provide a desired amount of protection from corrosion, or other deterioration of the underlying surface and substrate. For example, many surfaces that are painted are subject to environments that are conducive to corrosion, such a marine vessels that operate in water, which may include salt water. Further, motor vehicles and airplanes often operate in rainy or humid environments. Therefore, it is desirable to provide at least a minimum amount of protection against the corrosive effects of these environments.
In some applications, weight is an important consideration. For example, it is desirable to minimize weight of an airplane to reduce fuel consumption. Since the exterior surface area of the airplane may be significantly large, a paint and primer applied t the exterior surface can be a significant factor in the total weight of the airplane. Therefore, it may also be desired in some applications to limit thickness of paint or primer coating on a surface to a predetermined maximum thickness.
In order to ensure that a minimum, predetermined thickness of paint or primer coating is applied and that a maximum, predetermined thickness of a paint or primer coating is not exceeded, it would be desirable to accurately nondestructively determine the thickness of a paint or primer coating on a surface. Currently known nondestructive measurement techniques are limited in their applicability.
In one known method, eddy current testing is used to determine paint thickness on metal substrates. As is known, eddy current testing detects electrical currents, known as eddy currents that propagate within the metal substrate. As a result, eddy current testing can only be used to determine thickness of paint or primer that is coated onto a surface of a metal substrate.
In another known method, ultrasound testing is used to determine thickness of a paint or primer coating on a surface of a composite or plastic substrate. However, ultrasound testing is not as reliable as eddy current testing, and also exhibits reduced accuracy for coating thickness below approximately 0.0002 inches (2 mils).
Near infrared (NIR) testing has been successfully used to measure paint and primer thickness on a variety of substrate materials. However, NIR methods to date have been more difficult for paint or primer thickness of less than 2 mils and for shiny coatings. At least part of the problem in obtaining accurate and reproducible thickness measurements is due to specular reflectance from the coating that interferes with the NIR diffuse reflectance spectra.
Therefore, there is an unmet need to provide a system and method to accurately and nondestructively measure paint thickness regardless of the substrate material on which the paint is coated, and to obtain accurate measurements for thin coatings, especially coatings of less than 2 mils thick, and for coatings that have shiny surfaces.